This invention relates to apparatus for measuring boreholes and, more specifically, to calipers for measuring the diameter of a borehole.
It is known in the art to measure the diameter of a borehole with a caliper tool, which utilizes a sensing element such as a potentiometer. Typically, these devices utilize spring loaded arms to contact the borehole wall and have connected to the arms one or more elements of a potentiometer for determining the instantaneous borehole diameter.
The disadvantages suffered by this type of caliper include the exposure of the potentiometer wiring and parts to borehole conditions which can lead to corrosion and failure of these parts. This caliper also suffers from the disadvantage of having sliding electronic components that wear mechanically which can lead to inaccuracies and failure. Additionally, pressure terminals through which the electrical and mechanical lead-ins pass must be sealed off against pressure and corrosive fluids.
U.S. Pat. No. 4,205,266, issued to H. D. Lichtenberg and assigned to the assignee of the present invention, describes a borehole caliper intended to overcome the disadvantages of the potentiometer type caliper. This caliper utilizes a flux gate magnetometer for sensing the distance between the flux gate and a magnetic element. The magnetic element is slidably mounted on the caliper body and is attached by mechanical linkage to a borehole contacting arm.
While this caliper represented an improvement in the art of measuring borehole diameter, it has been less than totally successfully. The flux gate magnetometer can be effected by the earth's magnet field and by the magnetic field resulting from current flow in well casing. These effects present problems in accurately calibrating the caliper instrument outside the borehole. Further, the magnetic element can degrade due to exposure to the temperatures and pressure within the borehole.
These and other disadvantages are overcome with the present invention by providing an apparatus for measuring the diameter of a borehole which has no critical elements exposed to the wellbore.